The invention relates to transmission synchronizing apparatus and method of a modem for obtaining synchronization in the modem on the basis of a transmission timing signal which is given from a data terminal equipment and, more particularly, to transmission synchronizing apparatus and method to synchronize both of the modulating operation of a transmitting section and the demodulating operation of a receiving section by using the same internal timing signal.
When a frequency multiplex modem is used in a multipoint system (polling system) and a signal is transmitted synchronously with a transmission timing signal from the outside, the frequency multiplex modem of a master station obtains a transmission synchronization by synchronizing an internal timing signal of a baud rate for transmission with the transmission timing signal from the outside. Further, a reception synchronization is also obtained by the internal timing signal for transmission synchronization. Therefore, in order to synchronize the internal timing signal with the transmission timing signal, when a phase jump to coincide the phases of both of those signals is executed at the time of pull-in, there is a fear such that a reception data error is caused. Thus, it is necessary to use a new method of synchronization.
Hitherto, a multipoint system to execute a data transfer with a predetermined slave station by polling from the master station has been put into practical use in order to improve a use efficiency and a reliability of a line. The modem which is used in the multipoint system can synchronize the internal timing signal of the baud rate by the transmission timing signal from the outside with respect to the transmitting section. With regard to the receiving section, the internal timing signal of the baud rate is synchronized by extracting a timing signal from a carrier signal which was received.
The transmission timing signal from the outside is a timing signal of 2400 Hz, 4800 Hz, or 9600 Hz which is decided by a data transfer speed of 2400 bps, 4800 bps, or 9600 bps, or the like. The internal timing signal is a signal having a repetitive period (baud rate period T.sub.B) which is decided by a modulation speed, for example, 2400 bauds.
With respect to the case of 9600 bps, a transmission timing signal 106 is shown in FIG. 1A. An internal timing signal 110 of the baud rate of 2400 bauds, which is synchronized by a PLL activation, is shown in FIG. 1B. Further, a transmission request signal 100 is shown in FIG. 1C. FIG. 1D shows an internal timing signal 110' which is not synchronized, for the purpose of comparison.
It is now assumed that a baud rate is set to 2400 bauds, and in order to obtain a data transfer speed of 9600 bps, EQU 9600 bits/second.div.2400 modulation/second=4 bits/1 modulation
Therefore, the number of data signal points in a phase plane which is used for modulation is equal to 16.
In the conventional modem which dan independently obtain the synchronization between the internal timing signals of the transmitting section and the receiving section, the pull-in of the internal timing signal 110 to the transmission timing signal 106 is executed by a phase jump as shown in FIGS. 1A to 1D.
It is now assumed that the transmission timing signal 106 in FIG. 1A is not synchronized with the internal timing signal 110 in FIG. 1B, and that their phases are deviated. Such a phase deviation corresponds to a time .DELTA.T from the leading edge at time t.sub.12 of the internal timing signal 110' in FIG. 1D, as illustrated for comparison to the leading edge at time t.sub.2 of the transmission timing signal 106 of FIG. 1A.
When the transmission request signal 100 in FIG. 1C is turned on from off at time t.sub.1, the transmission timing signal 106 in FIG. 1A and the internal timing signal 110 in FIG. 1B are not synchronized at time t.sub.0 before t.sub.1. Therefore, the transmission internal timing signal 110 is phase jumped and pulled in so as to coincide with the leading edge of the transmission timing signal 106 at time t.sub.2 after the transmission request signal 100 was turned on, and a PLL operation (phase locked loop operation) is started. The phase jump illustrated in FIGS. 1A-1D shows that the leading edge rising at time t.sub.12 of the internal timing signal 110' in FIG. 1D jumps by .DELTA.T and is synchronized by the PLL operation at time t.sub.2, as shown in FIG. 1B. Therefore, the baud rate period of the internal timing signal 110 of FIG. 1B is initially increased by the out-of-synchronization amount .DELTA.T, thereby becoming a long baud rate period equal to T.sub.B +.DELTA.T. As illustrated by FIG. 1B, in the next baud rate period, the baud rate period is returned to the correct baud rate period T.sub.B by the PLL operation.
On the other hand, in recent years, a frequency multiplex modem using a plurality of carriers of a small roll-off rate having different frequency bands is used for a multipoint system in order to improve a use efficiency of a network. When the frequency multiplex modem of a slave station receives a polling from the master station, a data multiplex transmission can be executed between a plurality of data terminal equipment connected to the slave station and a host computer of the master station.
In such a multipoint system using the frequency multiplex modem, it is difficult to extract a timing signal by a modem demodulating section of the terminal from the reception signal of the carrier of a small roll-off rate. Therefore, the master station modem always transmits a carrier to a secondary channel and the slave station modem extracts a reception timing signal from the carrier reception signal of the secondary channel and further uses the extracted reception timing signal as a transmission timing signal.
In the master station modem, since the system has a multipoint construction, it is not expected that a carrier by the secondary channel from the slave station side is always received. Therefore, both of the transmitting section and the receiving section of the modem are synchronized on the basis of the transmission timing signal from the host computer or the transmission timing signal in the modem, thereby obtaining a synchronization of the system.
In the master station of the frequency multiplex modem which is used for the multipoint system, however, the internal timing signal for transmission is synchronized with the transmission timing signal from the outside and at the same time a synchronization of the receiving section is obtained by using the internal timing signal for transmission, so that when a phase control by the PLL is executed by pulling in the transmission timing signal from the outside by a phase jump of the internal timing signal as in a conventional manner, such a phase control becomes a cause to generate a reception data error.
For example, in the sync pull-in by the phase jump shown in FIGS. 1A to 1D, a time interval from time t.sub.0 to pull-in time t.sub.2 is longer than the correct baud rate period T.sub.B (1/2400 second), which is illustrated in FIG. 1D for the internal timing signal 110'. Accordingly, a sampling period of an A/D converter is also longer. Therefore, a carrier cannot be reproduced accurately, an error occurs in the reconstructed reception data, and a reception data error occurs.
In the slave station, it is similarly considered by replacing the external transmission timing in the master station to the reception timing extracted from the carrier.